^^ Sorry for wall of text, it was almost as long as a guide lol. Apologies for the doublepost as well.

I just typed up this longass comment and then it decided to delete it when I tried to submit.. so I'll try to recreate it to the best of my memory.

First off, why Teleport AND Boots of Mobility? I can't argue with having one or the other but both seems a bit like overkill, for this if you're gonna take tele you should switch them out for Boots of Swiftness instead (almost always better than mobility imho), or even for this build berzerkers might not be too bad (even though I hate those boots).

A quick question, though, about your starting items. What are you going to do if you're playing solomid and your opponent is completely shutting you down? You're getting harassed to no end, can't autoattack and can barely even try to last hit. In this situation vampiric sceptre is useless... Also why would you want to get lifesteal off the bat? You shouldn't be autoattacking creeps but should be focusing on getting last hits for teh moneyz. And don't forget, Twitch is extremely squishy early on so I tend to go for Doran's Shield + HP pot because it's safter and you always get the hp/5 no matter whether you get to last hit or not, I know it doesn't build into anything but it should help you get a better start which equals more gold anyway.

Alright one more thing, I'm quite wary about stacking AS on Twitch considering his natural AS boosts. You did pretty well with that in this guide but I'm really curious as to why you chose Malady as the AS item. Youmuu's Ghostblade is a fantastic item on Twitch and really seems to work with what you have going here, if you replace Malady with that sure you lose the pseudo-poison, but you already have poison. With Youmuu's you get more damage, some sweet armor pen, and then a more versatile ability where you can use the AS to help gank or use the increased movement speed to runaway! when a good gank goes bad...

Overall not a bad build I can see how it would be effective, nice choice on the Tiamat as Twitch is one of the few champions in the game who is effective with it. Feel free to comment back and I'll see what you have to say :o. I like the items, masteries aren't bad (might take that 1 out of Meditation and move it over to the Neutral Monster Buff duration since Lizard is your friend), and like the runes but it brings up an entirely different debate on chance vs dmg for critrunes. Good guide that could become the definitive Twitch guide with a little bit of work! Feel free to add more descriptions in there too, longer is better to help out teh nubs.